This invention relates to methods for controlling the structure of boron nitride thin films through the introduction during film growth of certain crystallographic defects.
Bulk boron nitride exists in three crystallographic modifications:
a) the hexagonal graphite structure
b) the cubic zincblende structure
c) the hexagonal wurtzite structure
The hexagonal graphitic structure is the thermodynamically stable phase with a hexagonal bonding exactly analogous to graphite. Because of the weak forces between the hexagonal planes this material can be used as a solid state lubricant.
The cubic and the wurtzitic structures are metastable phases which do not exist in nature. In the laboratory they have been synthesized by high pressure methods. The cubic structure has been formed by applying high static pressure (40,000-60,000 atmospheres) at relatively high temperatures (1400.degree.-1700.degree. C.) in the presence of solvent catalysts (alkali metals, alkaline earth metals, and their nitrides and borides). The wurtzitic structure has been formed by high pressure shock wave methods. These high pressure methods lead to the formation of powders, which subsequently are sintered under high pressure conditions to produce bulk materials for grinding and cutting applications.
The bonding in the cubic and wurtzitic structure is tetrahedral and the symmetry of this bond makes these two structures the second hardest materials in nature after diamond. The cubic and the wurtzitic structures have cubic and hexagonal symmetries because the identical planes of atoms in the two structures repeat themselves with a sequence ABC--ABC--ABC . . . and AB--AB--AB . . . respectively. However, the hardness of the two structures is identical since it is determined by the tetrahedral coordination, rather than by the crystallographic symmetry.
Boron nitride thin films have been grown by low pressure and temperature methods. The majority of these methods, including chemical vapor deposition and plasma enhanced chemical vapor deposition, lead to graphitic or amorphous boron nitride. Other methods, including reactive evaporation, ion beam implantation, reactive pulse plasma deposition and ion beam extraction have led to the deposition of the tetrahedral coordinated boron nitride compounds. In these methods, the film is deposited from ionic boron and nitrogen containing species.